1. Field of the Invention
The invention relates to power transmission belts including V-belts, multi-v-ribbed belts and toothed power transmission belts, and more particularly to such belts comprising a longitudinally extending tensile member comprising one or more cords, at least one of which being formed of a carbon fiber yarn, and to a toothed belt for providing piston-to-valve synchronization in automotive internal combustion engine, and to a method for reducing the angular vibration of a camshaft in relation to a crankshaft in an internal combustion engine in which the camshaft is driven by a toothed belt.
2. Description of Related Art
Power transmission belts are commonly used to transmit power as between pulleys. They may be subjected to extremes in temperature and in loading during normal operation. By virtue of their composite structure, comprising both relatively low modulus cured elastomer body portions and a relatively high modulus tensile member defining the primary load carrying component of the belt, and the extremes in loading and temperature to which they may be routinely put, a high degree of durability, flexibility, and consistency are required of each of the component parts.
One particular problem associated more commonly but not exclusively with the utilization of toothed belts is that of belt growth, which constitutes a permanent deformation of the belt, and can occur as a result of extended operation, extreme dynamic loading, extreme temperatures, improper component selection, or combinations of the foregoing. In particular, if the materials surrounding the tensile member are insufficiently heat resistant, operation at relatively high temperature can so embrittle those materials that they no longer effectively bond the tensile member to the surrounding belt body, leading to a drop in belt tensile strength and hence rapid belt growth. Such permanent deformation of the belt leads to improper tooth-groove interaction, tensile failure and ultimately to catastrophic failure of the tensile cord.
The introduction of carbon fiber as a reinforcement material in rubber composite articles has presented the possibility of improved performance for some applications due to its relatively high modulus compared to conventional fibers, e.g., glass cord. To date however, adhesion of the fibers to the surrounding elastomer component for extended operating lifetimes and related problems have not been adequately resolved. U.S. Pat. No. 5,807,194 discloses the use of carbon fiber as the tensile cord in a toothed power transmission belt construction possessing urethane belt body portions. That disclosure is limited to carbon fiber cord having a cord treatment, which allows for its incorporation within the belt composite structure, involving the cord picking up various amounts of the urethane belt material itself during the belt-casting process. The castable nature of the urethane material itself, i.e., its liquid form prior to a curing step, allows the urethane to flow about the carbon fibers and within the interstices thereof. The disclosure is inapplicable however to belt constructions involving non-castable elastomer belt body portions, e.g., hydrogenated acrylonitrile butadiene rubber (“HNBR”), and polychloroprene rubber (“CR”).
Toothed belts for use in driving a camshaft in internal combustion engines exemplified by automotive engines are faced with particular problems impacting their durability. The operating engine and each of its individual parts contributes vibration to the engine system, both as a result of the imbalance that exists between the rotating- and reciprocating engine parts, and of the angular oscillations of the shafts that are the result of the repetitive combustion impulses, i.e., the ignition of each of the cylinders, that create angular forces on the crank- and camshafts. Such vibration has been related to fatigue failure of timing belts. Particularly in the case of synchronous belts used to drive a plurality of components and/or in interference-type engines exemplified by diesel engines wherein there is very little clearance between the valve and piston, failure of the timing belt can cause severe and costly engine damage.